Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 93)

Fig. 3.6-1
Stator resistance soft starting for gentle motor starting
Note
A motor protective device without differential release must be used, as it would otherwise
operate during start-up.
3.7 Pole-changing motors
3.7.1 Speed change by pole changing
The number of poles determines the rated speed in asynchronous motors at a given supply
frequency. If the stator windings are designed for two or more different pole numbers, the speed
can be changed in just as many steps by switching-over.
The Dahlander circuit that with only one winding and six terminals supports two pole numbers
and hence speeds in the ratio 1:2 is especially economical. The rated powers and torques of the
two steps are thereby in a certain relationship to each other, depending on the circuit version.
The Dahlander winding is divided into individual windings groups, stepped according to the
smaller pole pitch. When current flows through each windings group in the same direction, the
higher pole number is generated, and when the current direction is reversed in each second
windings group, the lower pole number is generated. By repetition of the same windings
arrangement from pole to pole, a very good windings symmetry is achieved.
A special type of Dahlander circuit is the PAM circuit (pole amplitude modulation). In PAM
motors, an asymmetry of the field harmonics is accepted and the windings are grouped so that
the resulting pole numbers are in ratios other than 1:2 (e.g. 6/4-pole or 8/6-pole).
Motors in PAM circuits, like those in Dahlander circuits, only have six terminals. For both
winding types, the same versions of the external circuit diagram can be used. An additional star
point contactor is always required for the YY stage, in addition to the two feeding contactors of
both steps.
Pole changing can also be achieved by regrouping the windings branches. This is known as
phase mixing or phase modulation. In this case the winding along the periphery is divided into
coil blocks. Depending on the number of these blocks, double or multi-stage pole changing can
be performed. Three terminals are required per speed level.
With pole changing by phase modulation, the connection diagram provided by the motor
manufacturer should be consulted when selecting the external circuit layout and the switchgear.
Either only one feeding contactor (for example YYY/YYY circuit) is required per stage or in
addition a supplementary bypass contactor (for example
∆/∆∆ circuit).
In Tab. 3.7-1 and Tab. 3.7-2 a summary is provided of the most common arrangements and
circuit layouts of stator windings for pole-changing motors.
LVSAM-WP001A-EN-P - April 2009 3-17